This invention relates generally to optical elements and more particularly to coatings which protect and strengthen IR optical elements.
As is known in the art, optical imaging systems generally include an optical element which shields the remainder of the imaging system from a hostile environment. For example, with infrared (IR) airborne imaging systems, an IR transparent optical element, such as window or dome, is mounted on the airborne system to isolate the remainder of the IR imaging system from exposure to humid, corrosive, abrasive, and high temperature environments. Prolonged exposure to these environments generally degrades the optical and physical characteristics of the material of the optical element. For many applications involving low speed missiles, low velocity water droplet impact is generally the most severe environmental exposure. However for newer hypersonic missile applications, aerothermodynamic loading of the optical element is the most severe environmental exposure since it can lead to catastrophic failure.
Failure of the element, whether the result of water droplet impact or aerothermodynamic loading, results from the environment interaction with the surface of the external element producing subsurface fractures, even at subsonic velocities. For very brittle materials these subsurface fractures are initiated at pre-existent microflaws, the so-called "Griffin flaw," lying near the surface of the optical element. Damage to such optical elements occurs prior to any significant removal of material. The mere propagation of these pre-existent microflaws is sufficient to damage the optical element. In particular, these microflaws are propagated through the optical element by the tensile component of the surface stresswave created at the time of impact with the water droplet or excess aerothermodynamic loading. Once formed, the continued propagation of the subsurface fractures through the optical element will often produce large cracks in the optical element. In the region of the crack, scattering and refraction of incident IR energy occurs producing increased internal reflections and IR energy losses. With a significant number of such cracks, the transmissivity of the optical element is severely reduced. Furthermore, as cracks propagate through the optical element, catastrophic failure of the element will occur. When the optical element shatters or breaks, the remaining optical elements of the IR imaging system are exposed to the external environment, resulting in potential catastrophic damage to the imaging system.
Typically, materials which offer the best mechanical durability and optical performance for infrared imaging systems in the 8 .mu.m to 12 .mu.m infrared band, are limited to a relatively small number. Suitable materials include zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride and cadmium telluride. Ternary sulfide materials such as calcium lanthanum sulfide are also currently being developed for IR applications, particularly in the 8-12 .mu.m band. These ternary sulfide materials may provide some improvement in durability but even these materials are susceptible to the environmental exposures mentioned above. Generally, all of the aforementioned materials are relatively brittle and soft, and have low flexural strength, leading to low thermal shock resistance and resistance to damage, sustained during high velocity water droplet impact.
With the possible exception of polycrystalline diamond, which theoretically can provide an optical element having high strength and high transmittance, the best compromise materials currently available are zinc selenide (ZnSe) and zinc sulfide (ZnS) with ZnS being the more durable and ZnSe being the more transparent of the two. However, both materials are relatively soft and weak compared to materials used at midrange infrared wavelengths (i.e. 3 .mu.m-5 .mu.m).
It is also known that certain grades of zinc sulfide have extended optical transmittance ranges. For example, RAYTRAN.RTM., Multispectral Grade.RTM. ZnS from Raytheon Company is transmissive in the visible portion of the wavelength range. It, however, also has hardness and strength characteristics which are lower than standard RAYTRAN.RTM. zinc sulfide.
Accordingly, a technique which can strengthen and harden such 8 .mu.m-12 .mu.m materials, particularly zinc selenide and zinc sulfide without affecting optical properties in any significant manner would be highly desired.
It is known in the art that high modulus coatings, such as a layer of hard carbon having quasi-diamond bonds and substantial optical transparency, when provided over germanium provides limited protection to germanium optical elements from impact damage caused by rain erosion and abrasion. Hard carbon coatings on germanium are described in an article entitled "Liquid Impact Erosion Mechanisms In Transparent Materials" by J. E. Fields et al., Final Report Sept. 30, 1982 to Mar. 31, 1983, Contract No. AFOSR-78-3705-D, Report No. AFWAL-TR-83-4101. The hard carbon surfaces have not successfully adhered directly to other IR materials such as zinc sulfide and zinc selenide. Furthermore, hard carbon coatings even on germanium as mentioned in the article are susceptible to debonding during high velocity water droplet impact. A further problem with hard carbon is that the index of refraction of hard carbon is about 2.45, which is substantially higher than the index refraction of many of the aforementioned optical materials, such as zinc sulfide and zinc selenide. Accordingly, if an optical element is coated with a hard carbon coating, reflection losses at the incident surface of the optical element will be higher than if the optical element was not coated.
As also mentioned, most materials which are suitable for IR transparent windows, particularly in the 8 .mu.m-12 .mu.m band have low flexural strengths. This characteristic is particularly important in high aerothermodynamic applications of these elements where the element is under some static or dynamic mechanical load. In an article entitled "Direct Synthetic/Fabrication and Surface Modification of IR Window Materials For the 8-14 Micron Range," Annual Report No. 1, period Mar. 1, 1985-Feb. 28, 1986, Contract No. N00014-85-C-0140, Task II, Korbin et al pgs 5 - 7, the authors describe their work in which single compressive layers of Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, and AlN were deposited over a glass base by reactive ion beam deposition. Their measurements indicate that the deposited material had an intrinsic compressive stress. Their attempts to extend the work to LWIR materials, such as ZnS were futile. In another article entitled "Impact Damage Threshold in Brittle Materials Impacted by Water Drops" by A. G. Evans et al., Journal of Applied Physics 51 (5), pps. 2473-2482 (May, 1980) at page 2481, it was theorized that martensite toughening (phase changes) at the surface of the brittle material may be useful in tempering such brittle materials. It was also theorized that surface compression stresses could be of benefit. However, the authors gave no specific description what they meant by "surface compression." These brittle materials undergo surface compression when incident water drops impact the surface of the material.